1. Field of the Invention
The present invention relates generally to an apparatus for sensing a force exerted by one body or fluid upon a surface. More particularly, the invention relates to an instrument for measuring pressure external to the instrument by sensing the force exerted upon a flexible diaphragm, the instrument using a vibratory element for measuring changes in density of a working fluid contained in the instrument which is responsive to movement of the diaphragm and therefore the pressure being measured.
2. Description of the Prior Art
For many years it has been increasingly important to quickly and accurately measure pressure, particularly the pressure of a fluid. Recently, it has been disclosed that a vibratory element, such as a quartz crystal, may be used to measure fluid density and thereby measure parameters which may cause the density of the fluid to change such as pressure, temperature or acceleration.
An example of this technology is disclosed in U.S. Pat. No. 4,526,480, issued to Ward. Referring specifically to FIG. 3 of Ward, there is illustrated a fluid density/pressure transducer having a hemispherically shaped housing and a flexible diaphragm sealingly mounted over an opening in the housing. Within the housing is mounted a vibratory element or tuning fork surrounded by working gas sealed within the internal chamber of the housing. Connected to the tuning fork is an oscillator circuit for causing the fork to vibrate and a display to provide a visual indication of the frecuency and therefore the pressure being observed. As the external pressure is changed, the diaphragm is caused to move inwardly into the housing with a pressure increase and outwardly from the housing with a pressure decrease. Therefore, with a changing pressure, the density of the captured working gas within the housing chamber changes. Since the frequency of resonation of the tuning fork varies in proportion to any change in the density of the working fluid surrounding the fork, the frequency observed by the display provides a highly accurate indication of the amount of pressure being exerted upon the diaphragm.
It has also been observed in the art that the range of frequency change exhibited by the fork and therefore the pressure measurement range of a device such as shown in FIG. 3 of Ward is dependent to a large extent upon maximizing the Compression Ratio. The Compression ("CR") being: EQU CR=V.sub.1 V.sub.2
Where V.sub.1 is the volume of the internal space in the housing chamber when the diaphragm is exposed to minimum external pressure and V.sub.2 is the volume of the internal space in the housing chamber when the diaphragm is fully flexed and exposed to the maximum external pressure. For this reason, prior art devices have attempted to minimize V.sub.2, the volume of internal space in the housing chamber when the diaphragm is fully flexed.
Heretofore, prior art devices have attempted to shape the chamber housing in such a manner that the diaphragm, when fully flexed into the housing chamber, contacts and conforms to the interior surface or backup surface, of the chamber. For example, the device shown in FIG. 3 of Ward employs a hemispherical interior wall or backup surface against which the flexible diaphragm is forced upon maximum deflection.
As can be appreciated, such prior art devices are highly dependent upon one being able to accurately predict the shape of the flexible diaphragm upon maximum flexure and then fabricating the interior wall or backup surface of the housing chamber to accurately correspond to that predicted shape.
While such devices may be generally acceptable for some intended purposes, they have not proven to be entirely satisfactory in that it is very difficult to accurately predict the diaphragm shape upon full flexure. This is especially so given the wide variations in materials used for flexible diaphragms and in the methods and conditions under which the diaphragm is sealingly mounted in place on the housing.
An additional problem faced by the prior art attempts to minimize the volume of the internal space at maximum diaphragm flexure is the difficulty and expense in fabricating such a contoured curved or backup surface. Such backup surfaces are often times complex and expensive to manufacture.
Additionally, it has been recognized in the art that it is desirable that the surface finish of the backup surface be smooth and substantially free from gas entrapping irregularities. At a minimum, it is believed that the surface finish of the backup surface should be as smooth as the diaphragm surface itself. However, it often is difficult and expensive to obtain such a smooth, polished surface when the backup surface is a curved surface. Otherwise, an excessive amount of working gas may not be fully compressed upon full flexure of the diaphragm. This failure to maximize compression and the density of the working gas observed by the tuning fork results in decreasing the pressure range available from the particular instrument. Again, smooth surface finishes can be difficult and expensive to obtain during normal manufacturing procedures when the surface is curved.
As a result of the short comings of the prior art, there has developed and continues to exist a substantial need for an accurate pressure sensing device using a vibratory element that has a greater pressure measurement range while being easy to manufacture. Despite this need, an accurate pressure sensor capable of measuring pressure over a wide range has heretofore been unavailable.